Lightly grease a 9 x 13 pan or a 9 inch square pan with a pad of butter or margarine.
Mix margarine, sugar and milk in a heavy 2 1/2 or 3 quart sauce pan. Bring to a full boil on medium heat, stirring constantly with a wooden spoon. Continue boiling and stirring for 5 minutes on medium heat. (You must stir constantly to prevent scorching).
Remove from heat. Stir in chocolate chips until melted. Add peanut butter, marshmallow cream and vanilla. Mix well and pour into pan. Cool at room temperature and/or refrigerate. Cut into squares.